1. Field
The subject invention relates to power distribution systems and in particular to a scalable power distribution system.
2. Related Art
Power distribution systems are typically used in facilities to convert transmitted high voltage energy to energy that is suitable for its intended use and deliver that energy within the facility. These facilities (e.g., hospitals, data centers, etc.) have a load (e.g., computers, heating and air conditioning equipment, etc.) to which the energy is delivered by the power distribution system. An exemplary power distribution system that is used in these facilities is a distributed redundant power system.
In a distributed redundant power distribution system, N+1 redundancy is achieved by providing two sources of power to a single load (e.g., servers, chillers, etc.) from two diverse, totally independent sources. When there are only two such sources, there is only one combination (A+B). The system, therefore, has 2N redundancy. When the system has three sources (A, B and C), there are three combinations of two: A and B, B and C and A and C. Similarly, when there are four sources (A, B, C, and D), there are six combinations of two: A and B, B and C, A and C, A and D, B and D an C and D. When there are five systems, there are ten combinations; and, when there are six systems, there are sixteen combinations.
Loading of the system using multiple sources can be as high as (1−1/N) times the total system capacity without overloading any system in the event of a single source failure. The criteria for achieving this maximum limit are that every possible combination of two systems needs to provide two-source power to an equal amount of load. For example, for five sources, there are ten load blocks, each of which needs to supply two source loads of 10% of the total load served. The total load can then be as high as 80% of the total system capacity.
For five 675 KW sources, for example, the total capacity is 3375 KW which would yield 2700 KW of distributed N+1 capacity as long as each of the ten combinations of two sources is loaded to 270 KW, split evenly between the two sources. This configuration and loading would put a normal load of 540 KW on each source. Failure of any source causes the paired source in each of the four two-source combinations with that failed unit to assume half the 270 KW supplied by the two-source combination. This load assumption raises each of the four remaining sources from 540 KW to their maximum capacity of 675 KW.
Implementation of a full system design as described above is relatively straightforward if N is a known number and the entire system is built before the critical load is connected. In practice, however, economic forces encourage the construction and placement in service of small systems that grow over time. Typically, initial construction includes two UPS modules (UPSA and UPSB), each rated 675 KW full load. For this configuration, all two-source loads are fed from UPS A and UPS B and the total capacity of the system is half of the connected capacity of 1350 KW (675 KW). When a third UPS is added, there are three possible combinations of two source loads, as described above, bringing the total capacity of the system to 1350 KW (450 KW each). When a fourth UPS is added, there are six possible combinations of two-source loads, as described above, bringing the total capacity of the system to 2025 KW (337.5 KW each). When a fifth UPS is added, there are ten possible combinations of two-source loads, as described above, bringing the total capacity of the system to 2700 KW (270 KW each).
The trend in the above example is for the total capacity of each two-source load combination to decrease as the overall system capacity increases. However, to use all of the available power with the two sources, ⅓ of the available power is planned to be moved to implement full capacity when the third UPS is added; then, ¼ of the load is planned to be moved to implement full capacity when the fourth UPS is added; and ⅕ of the load is planned to be moved to implement full capacity when the fifth UPS is added.
Furthermore, when the third UPS is added to the system, the load combination may need to be changed. For example, the B connection may need to be changed to C to make it an A and C load to balance the system. When the connection is changed, the B connection is removed and the circuit is rerun or rerouted and connected to C. A problem with changing the connection is a loss of redundancy during the transition. For example, as soon as B is disconnected, the load has only one source. If the end devices are truly two-source, this may be no problem at all. However, if the A source fails while the B source is being changed to C, this reduction in redundancy results in a critical load interruption. In many cases, this risk of critical load interruption is unacceptable. In addition, the configuration prevents full capacity utilization at earlier stages of construction and presents a construction sequence level of difficulty that drives costs up and significantly delays full implementation.